The present invention relates to oscillator circuits and more specifically to microwave oscillator circuits.
Hybrid technology and semiconductor monolithic processes have been used to fabricate a wide variety of microelectronic components, circuits and systems with limitations as to the type of components that can be conveniently and economically manufactured. Devices such as inductors or capacitors usually require a large substrate area because these devices are usually specified with relatively high capacitive or inductive values which can only be realized when the devices have large physical geometries. Generally, large physical devices decrease the yields of hybrid and semiconductor processes.
Physically large devices also have correspondingly large intrinsic parasitic parameters which cause large component tolerances and which tend to reduce the operating frequency and tend to reduce the predictability of the operating conditions of the circuits employing the large devices. As such, large physical devices are avoided when designing hybrid or monolithic semiconductor microwave electronic circuits which may operate in the 1 to 10 GHz range.
Heretofore oscillator designs for RF and microwave circuits have incorporated discrete active elements such as bipolar transistors. Typically, these oscillators have a single active transistor coupled to discrete inductive-capacitive resonating components.
For example, a bipolar transistor with an inductor connected across its base-collector junction, a capacitor connected across its base-emitter junction, and a second capacitor connected across its emitter-collector junction is such a resonant oscillator. This bipolar oscillator requires large capacitive and inductive values. The hybrid or semiconductor manufacturing process used to fabricate this design suffers from poor yields and high fabrication costs.
Furthermore, a problem with bipolar semiconductor technology is that it generally requires more power than, for example, a corresponding circuit processed by metal-oxide-semiconductor technology using field effect transistors (FETs). Moreover, the bipolar resonant oscillator using a single active component also produces harmonics which may be coupled into surrounding circuits as unwanted noise.
A single active transistor oscillator may comprise a FET amplifier coupled to cascaded capacitor-resistor networks. A conventional design of a FET amplifier may have the source of the FET connected to ground while the drain of the FET is connected to a positive power supply through a drain resistor. This FET amplifier of conventional design is then followed by a plurality of cascaded arrangements each having a capacitor (C) and a resistor (R) with the input of the cascaded RC networks connected to the drain and with the output of the last RC network being fedback to the gate of the FET. This FET amplifier shifts by 180 degrees the phase of any voltage which appears on the gate respecting the drain while the network of cascaded RC networks shifts the phase by an additional amount. At some frequency, the phase shift introduced by the cascaded RC networks will be precisely 180 degrees, at which frequency, the total phase shift from the gate to the drain through the cascaded RC network will be zero module 360 degrees). This particular frequency will be the one at which the circuit will oscillate. This single active FET oscillator again suffers from requiring relatively large capacitive values with correspondingly poor process yields and with a reduced and unpredictable oscillating frequency.
Typically, single active resonant oscillators and single active FET oscillators require capacitors or inductors having large inductive and capacitive values which dictate that these components disadvantageously have relatively large physical sizes. Hence, the heretofore single active component oscillator circuits suffer from poor yields, imprecise and low oscillating frequencies, the generation of harmonics and unwanted noise and large physical geometries. These and other disadvantages are solved using cascaded resistive feedback active FET amplifiers in a zero-phase-shift-loop microwave oscillator.